1. Field of the Invention
This invention relates generally to a color signal processing circuit in a color video tape recorder (hereinafter called VTR), and more particularly, to a color signal processing circuit for correcting the phase of a color signal, which includes phase fluctuating components, when a video signal is reproduced by a VTR.
2. Description of the Related Art
FIG. 19 of the accompanying drawings shows a conventional color signal processing circuit disclosed in, for example, a magazine "NHK Home Video Technology" (pages 83-86) edited and published by Nippon Hoso Kyokai. In FIG. 19, reference numeral 1 designates an input terminal for a low-frequency-range color signal; 2, a low-pass filter (LPF) for removing unnecessary components other than the low-frequency-range color signal; 3, a first frequency converter, which is a multiplier, for converting the low-frequency-range into an initial frequency; 4, a first band-pass filter (BPF) for removing unnecessary components from an out-put signal of the first frequency converter 3; 5, a burst separator for separating color burst signal components from the output signal of the first band-pass filter; 6, a reference signal oscillator for generating a reference signal which is to be a phase reference of the output of the first band-pass filter; 7, a phase detector for performing phase comparison between the output signal of the burst separator 5 and the output of the reference signal oscillator 6; 8, a voltage control oscillator (hereinafter called VCO) which is controlled by the output of the phase detector 7; 9, a second frequency converter, which is a multiplier, for producing a product of the output signal of the reference signal oscillator; 10, a second band-pass filter (BPF) for removing unnecessary components of the output signal from the second frequency converter 9; and 11, an output terminal for a color signal.
The operation of the conventional circuit will now be described.
A low-frequency-range f.sub.L including a frequency shift +.DELTA.f created due to a nonuniform feed of a tape, irregular rotation of a rotary drum or any other cause is inputted to the low-pass filter 2 from the input terminal 1. The low-pass filter 2 removes unnecessary frequency components from the inputted low-frequency-range color signal. An output signal of the low-pass filter 2 is supplied to a first frequency converter 3 and is thereby converted into an original color signal carrier frequency f.sub.sc. For example, in the NTSC method, a frequency of 3.58 MHz is selected. The first frequency converter 3 creates many high frequency components other than necessary color signals, while the first band-pass filter removes unnecessary components. An output signal of the first band-pass filter 4 is supplied to the output terminal 11 to be a color signal output signal and is also supplied to the burst separator 5 only a color burst signal is extracted. An output signal of the burst separator 5, together with an output signal of the reference signal oscillator 6 which generates a reference signal f.sub.SC of 3.58 MHz, is supplied to the phase detector 7 where a phase difference between these two signals is detected.
Then an output of the phase detector 7, namely, a phase error signal controls the VCO 8 so as to generate a frequency f.sub.L equal to the low-frequency-range signal, which is inputted to the input terminal 1, plus/minus .DELTA.f. An output signal of the VCO 8, together with an output signal of the reference signal oscillator 6, is supplied to the second frequency converter 9 where a product of these two signals is produced. An output signal of the second frequency converter 9 includes frequency components f.sub.SC +F.sub.L .+-..DELTA.f and f.sub.SC -f.sub.L .+-..DELTA.f, and therefore, only the components f.sub.SC +f.sub.L .+-..DELTA.f are extracted by the second band-pass filter 10. The output signal of the second band-pass filter 10 determines a carrier frequency of the first frequency converter 3. As an output signal of the first frequency converter 3, a signal having frequency components f.sub.SC and f.sub.SC +2(f.sub.L .+-..DELTA.) is obtained, but the component f.sub.SC is extracted by the first band-pass filter 4. Here the first frequency converter 3, the first band-pass filter 4, the burst separator 5, the phase detector 7, the VCO 8, the reference signal oscillator 6, the second frequency converter 9 and the second band-pass filter 10 jointly constitute a phase locked loop (hereinafter called "PLL circuit"), in which a closed loop is included such that the color burst signal phase of the output signal of the first band-pass filter 4 is normally synchronized with the phase of the output signal of the reference signal oscillator 6.
The operation characteristic of an ordinary PLL circuit will now be described with reference to FIG. 20.
In FIG. 20, a signal of .THETA..sub.1 (s) (s: Laplace operator) is inputted to an input terminal 15 and is supplied to the phase detector 7. The phase detector 7 performs phase comparison between the phase .THETA..sub.1 (s) of the input signal and a phase .THETA..sub.0 (s) (described below) of the output signal of the VCO 8 to obtain a phase difference and outputs a voltage according to the phase difference. Here the ratio of output voltage with respect to the phase difference of the phase detector 7, namely, a conversion constant is represented by K.sub.d. The output signal of the phase detector 7 includes many high frequency components. Therefore, if these high frequency components are removed and a closed loop is incorporated, the output signal of the phase detector 7 is supplied to a loop filter 16 to control the response of the system. A transmission function of the loop filter 16 is represented by F(s). An output signal of the loop filter 16 controls the oscillating frequency of the VCO 8. The output signal of the VCO 8 is feedbacked to the phase detector 7 to form a closed loop. Although the oscillating frequency of the VCO 8 is controlled according to the input voltage, a phase difference is detected in the phase detector 7 so that the transmission function of the VCO 8 will be 1/S. Further, assuming that the ratio of the output frequency change with respect to the input voltage change is K.sub.0, the whole transmission function of the VCO 8 can be represented by K.sub.0 /S.
Then the change of output signal phase .DELTA..sub.1 (s) of the VCO 8 when the phase .THETA..sub.1 (s) of the input signal, namely, the transmission function H(s) of a closed loop is obtained by the following equation: ##EQU1## Here if the transmission function F(s)=(S.tau..sub.2 +1)/S.tau..sub.1 (where .tau..sub.1 =CR.sub.1 and .tau..sub.2 CR.sub.2) of a usual active filter shown in FIG. 21 is substituted for the transmission function F(s) of the loop filter 16, the transmission function H(s) of a closed loop can be expressed as follows: ##EQU2## where K=K.sub.0 .multidot.K.sub.d .multidot.Now assuming that ##EQU3##
(.omega..sub.n : natual angular frequency, .xi.: damping coefficient), the above equation will yield as follows: ##EQU4##
Then if the relation S=j.omega. is substituted for the equation (1), it yields as follows: ##EQU5## FIG. 22 is a characteristic graph showing frequency responses with the y-coordinate for the absolute value .vertline.H (j.omega.) .vertline. of H (j.omega.), and the x-coordinate for .omega./.omega..sub.n. As is apparent from FIG. 22, the frequency characteristic varies remarkably depending on the damping characteristic .xi.. When the angular frequency .omega. exceeds then actual angular frequency .omega..sub.n, the frequency characteristic attenuates along a curve of 6 dB/oct, thus resulting in a characteristic hardly responsive to high frequency. Even if the frequency characteristic is extended remarkably by increasing .xi., there would be no room for the phase of the system so that the loop will become nonstable.
Further, assuming that as a coefficient, the error of phase .THETA..sub.0 (s) of the output signal of the VCO 8 with respect to the input signal phase .THETA..sub.1 (s), namely, ##EQU6## is defined, the equation (3) will be ##EQU7## Further, if the relation S=j.omega. is substituted for the equation (4), this equation will yields as follows: ##EQU8## FIG. 23 is a frequency response with the y-coordinate for the absolute value .vertline.1-H(j.omega.).vertline. of an error coefficient and the x-coordinate for (.omega./.omega..sub.n), when the value of the damping coefficient .xi. is 0.707 as usual. As is apparent from FIG. 23, if the angular frequency .omega. is about 1/10 of the natural angular frequency .omega..sub.n, the error coefficient is -40 dB and hence substantial perfect responses. If .omega. is equal to .omega..sub.n, the error also will be about -3 dB, causing a considerable error. In the usual home VTRs, since .omega..sub.n is about 1800 (rad/s), .omega. which can secure an error coefficient larger than -30 dB will be about 360 rad/s, i.e., about 60 Hz in frequency.
A carrier color signal is obtained by amplitude-modulating the two color-difference signals (R-Y) and (B-Y) respectively by two perpendicularly crossing carriers cos .omega.ct and sin .omega.ct and by adding these two resulting color-difference signals. The carrier color signal is represented by EQU (R-Y) cos.omega.ct+(B-Y) sin .omega.ct.
FIG. 24(a) is a vector diagram of this carrier color signal. The burst signal includes no (R-Y) component, and the phase of the burst signal is equivalent to inverted sin .omega.ct.
With such carrier color signal, the phase of a color burst signal varies with respect to the reference signal. This means, as shown in FIG. 24(b), that the amplitude of the color-difference signals after demodulated vary according to the phase change .THETA., namely, a phase change is created.
When the output frequency of the VCO is followed to the input frequency by using a PLL circuit, the response characteristic to the phase change deteriorates to increase the error, causing nonuniform color tone appearing on the TV screen.
With the foregoing conventional color signal processing circuit, the response characteristic to a quick phase change of an input signal is not sufficient and, as a result, the remaining phase error component of the color signal would be nonuniform color tone and horizontal dragging noises, thereby deteriorating the quality of the color signal.